1. Field of the Invention
The present invention relates to earth boring arts and devices for subterranean well completion. In particular, the invention relates to fluid sealing elements for axially translated tubular members that are dynamically operative in chemically hostile, high pressure environments such as cross-over flow assemblies.
2. Description of Related Art
Deep subterranean wells are drilled to recover economically valuable fluids such as natural gas and crude petroleum. In many cases, these wells penetrate geologic zones that confine extreme pressure. When encountered, high pressure must be controlled and maintained. For those reasons, high pressure zones are often isolated by packers and casing liners.
Completion of a well for extended fluid production includes many complex procedures. Among these procedures are formation fracturing, gravel packing and cementing. To execute some of these procedures, it is necessary to redirect the normal course of well working fluid. Well working fluids are generally characterized as “mud” but the term expansively includes water, solvents and particulate mixtures for gravel and sand packing. The normal working fluid circulation route in a well starts from the fluid pumps at the surface. The fluid pump discharge is piped into the well tubing string (or drill pipe) and down the central flow bore of the tubing string. The working fluid exits the flow bore at the bottom of the tubing string or at a desired intermediate point through selectively opened apertures in the tubing wall.
In many cases, an annular space exists between the tubing string and the well bore or casing wall. This annular space, characterized as the annulus, provides a channel for the working fluid return flow stream to the well surface and back to fluid reservoir pits or the pump suction.
There are many circumstances in the life of a well that require fluid and pressure isolation of an axial section of the annulus from an adjacent section. For example, it may be desired to isolate the bottomhole pressure from the upper annulus and at the same time, shield a bottomhole production zone from contamination fluid entering the well bore from a higher strata. This desire is satisfied by means of a “packer” that provides an annular barrier between the tubing string and the well wall. Difficulty arises, however, when other well operations below the packer require fluid circulation. The packer blocks the working fluid return flow channel. This difficulty is overcome by means of a “cross-over assembly” which provides an additional flow annulus between the tubing string inner flow bore and an inner bore of the packer. Depending on the cross-over assembly setting, fluid may be pumped down the tubing flow bore into the annulus below the packer. From the annulus below the packer, the well working fluid (mud) may be channeled into the cross-over inner annulus to by-pass the packer and back into the well annulus above the packer.
In another typical example, working fluid may be pumped down the tubing flow bore to a cross-over point below the packer but above the tubing string bottom end. At the cross-over point, the working fluid is channeled out into the lower well annulus below the packer. The working fluid flows down the lower annulus to the bottom end of the tubing string to enter the tubing string flow bore. Working fluid flow from the bottom end of the tubing string is up the flow bore to the cross-over point where the flow enters the cross-over inner annulus. Further upward fluid flow proceeds along the inner annulus to a point above the packer where the flow is channeled out into the upper well annulus to complete the return flow circuit.
Typically, a cross-over assembly comprises an outer tube to which the packer is secured. Below the packer, the outer tube may have apertures through the tube wall. These outer tube apertures are internally isolated by reduced I.D. ring sections having smoothly finished sealing surfaces along the I.D. wall face of a ring section. A cross-over tube secured to the end of a tubing work string, has an O.D. less than the sealing surface of the outer tube ring section I.D. O.D. ring seals along the length of the cross-over tube are dimensioned to cooperate with the sealing surface I.D. Depending on the relative axial alignment between the cross-over pipe and the outer packer tube, controlled circulation flow past the packer is achieved while maintaining a pressure differential across the packer and ring seals.
Due to the hydrostatic head and bottom hole temperature of certain wells, this pressure differential across the packer and ring seals is considerable and imposes great pressure loads against the seals between the cross-over pipe and the sealing surfaces in the outer packer tube. These hydrostatic pressure and geothermal stresses may be compounded by an extremely hostile chemical environment. For example, a well may be treated with amine corrosion inhibitors to reduce the corrosive deterioration of the casing and production tube. Compounds such as zinc bromide may be used for well pressure containment and control. Additionally, inhibitor and containment compounds may be used in mixed combination. Both such well treatment compounds have aggressive consequences on the elastomers and polymers normally used to seal the tubular interface of completion and production equipment.
To resist these highly reactive well treatment compounds, special purpose sealant compounds such as fluoroelastomers based upon alternating copolymers of tetrafluoroethylene and propylene (AFLAS®) have been developed. However, these AFLAS® types of sealing compounds do not bond (Vulcanize) well with metallic substructures. When the high pressure load on a seal is abruptly released over the seal, such as when the cross-over tube is axially shifted, a resulting rush of fluid across a seal tends to dislodge and damage the seal.
It is an objective of the present invention, therefore, to provide reliable bore sealing elements for substantially coaxial tube members.
Another object of the invention is a highly improved cross-over assembly seal.